In the manufacture of high-density information discs, a diamond cutterhead is used to cut a surface relief pattern including information bits and tracking aids into a metallic substrate. The cutterhead assembly includes a diamond stylus bonded to a piezoelectric element, a pedestal member and, in turn, a mounting bracket.
As disclosed in U.S. Pat. No. 4,035,590 to Halter, herein incorporated by reference, the diamond stylus is bonded to the piezolectric element by means of an epoxy resin. At the present time the metallic substrate is cut at one-half real time; that is, it takes two hours to cut information into the substrate that represents one hour of playback time. As is readily apparent, if the substrate could be cut at real time rates, a substantial savings in time and costs would be realized. However, at such high recording speeds the temperatures generated in the piezoelectric transducer operating at real time frequencies up to 5 Megahertz rise rapidly to about 150.degree. C., as contrasted to operating temperatures of about 60.degree. C. at one-half time rates. All conventional commercially available high-temperature epoxy bonding materials tested resulted in failures between the diamond-piezoelectric element interface when tested at 150.degree. C.
The requirements for a bonding material that bonds the diamond stylus to a ceramic piezoelectric element at temperatures of about 150.degree. C. are most stringent. The bond must withstand the stress of curing high-density information patterns into metal, e.g., copper, for at least one hour of playback time. The bonding material must have a high heat deflection temperature (over 150.degree. C.) and high glass-transition temperature in order to provide adequate shear strength, tensile strength and lap shear strength at cutting temperatures. It must also be able to be cured at temperatures of 150.degree. C. or less to prevent damage to the piezoelectric element and in a layer of very uniform thickness across the diamond base to prevent undue stress and component bond failures at the piezoelectric-bonding material interface. The dimensions of the diamond stylus base are only about 150.times.150 microns; therefore, any non-uniformity in the thickness of the bond line results in high concentrations of stress in the layer, causing bonding failures. In addition, the bond strength is desirably over 3,000 psi at elevated temperatures, preferably about 5,000 psi, in order to give a useful life to the cutterhead assembly which must cut millions of information bits for an hour or more of playback time.